In order to construct an optical communication system using a multicore fiber for a transmission line, an optical amplifier that amplifies communication light is essential. For a typical optical amplifier, it is widely known to use an erbium doped fiber (EDF).
On the other hand, Patent Document 1 below is proposed as a multicore EDF. The multicore EDF described in this Patent Document 1 is configured for use for the purpose of the amplification of laser light.